The present invention relates generally to hollow-electrode switches with opposing anodes and cathodes at a spacing a and forming discharge gaps. More specifically, the present invention relates to such a switch having a trigger device comprising a hollow electrode associated with the discharge gap that is disposed in an ionizable gas filling, wherein the pressure p of the gas and the electrode spacing d at the discharge gap are selected so that the breakdown voltage of the gas discharge drops with an increasing product of pressure p and electrode spacing d.
The breakdown voltage for a given gas-discharge gap and its usual graphic representation as a function of the product of gas pressure p and electrode spacing d in the breakdown curve is known to be an important aid for the characterization of electric-discharge devices, with due allowance being made for the probability of breakdown. In determining the dielectric strength of a given gas-discharge gap, an infinitely large plate capacitor and its breakdown characteristic are generally used for comparison purposes. However, the practical embodiment of such discharge gaps involves electrodes with finite dimensions. To determine the right branch of the breakdown curve (Paschen curve), the so-called far breakdown region, including the voltage minimum, it will suffice to arrange two plane, rounded plates, which optionally may be provided with a so-called Rogowski profile at the edges, parallel to each other. However, such an arrangement will be useless for the investigation of breakdown characteristics in the left portion of the Paschen curve, i.e., in the so-called near breakdown region, because shunted discharges may then occur. Such shunted discharges can be avoided by means of an electrode construction with plane plate electrodes which are arranged coaxially to each other, are bent away from each other at their edges with a radius of curvature that is small in relation to the electrode spacing, and are guided along the inner cylindrical insulator surface. Between the bent, cylindrical edge region of the electrodes and the inside wall of the hollow cylindrical insulator, a gap is thus always formed. With this embodiment of a low-pressure gas-discharge gap, the breakdown curve for various inert and molecular gases, for example, can be determined also in the near breakdown region, i.e. to the left of the minimum of the Paschen curve, as disclosed in the Proceedings of the Seventh International Conference on Phenomena in Ionized Gases, Beograd, vol. 1 (1965), pp. 316-326.
Gas-discharge switches are also known which are controlled through a pulsed low-pressure gas discharge. They will switch currents of 10 kA, for example, with a voltage of 20 kV. These discharge switches comprise an anode and a cathode which are provided with coaxial openings and are separated from each other at the edge by an annular insulator. For the gas discharge, a control device is provided that comprises a hollow electrode which is configured as a cage and is electrically connected to the cathode and thus is at cathode potential. It encloses the rear space of the cathode and separates it from a pre-ionization area. The gas discharge between cathode and anode is initiated by the injection of charge carriers. The discharge is started in two stages. First a pre-ionization is produced by an auxiliary electrode through a glow discharge outside of the hollow electrode. Then a negative pulse is applied to a trigger electrode and the penetration of charge carriers into the hollow electrode is made possible by switching the potential of a blocking electrode to zero. The discharge is initiated with the penetration of the charge carriers. This gas-discharge switch is relatively complicated. This is disclosed in J. Phys. E: Sci. Instr. 19 (1986), The Institute of Physics, Great Britain, pp. 466-470.
Another known design of a hollow-electrode switch in which the hollow electrode is electrically connected to the cathode comprises a cathode and an anode, each of which is provided with a central bore. A discharge gap is formed between these bores. The distance between the electrodes, which in the discharge region are arranged parallel to each other, is greater than in the channel formed between the electrodes outside of the discharge region in the radial direction. This is disclosed in German published patent application OS 37 21 529, FIG. 30.
The gas-discharge switch may also comprise a plurality of discharge channels which are provided with a trigger device that is common to all of them. This triggering device comprises a common hollow electrode which is electrically connected to the common cathode. The synchronous discharge in the discharge channels is initiated by charge carriers which penetrate from a pre-ionization region into the cathode rear space through holes in the bottom of the cage.
In this known design, the anode and the cathode are provided with a recess at each individual discharge gap, and the facing surfaces of anode and cathode therefore are not parallel to each other in the discharge region. The discharge gap is formed also in this arrangement between the central bores. This is shown in FIG. 13 of WO 89/10646.
The dielectric strength of the switch with constant gas pressure is essentially influenced by the electrode spacing, the diameter of the openings in the electrodes, the thickness of the electrode material, and hence the depth of the holes. The diameter of the openings has been found to be particularly critical since on the one hand high dielectric strength calls for a small diameter while on the other hand reliable triggering requires a predetermined minimum diameter. During the switching of high currents, the electrode material in the peripheral area of the openings is worn away with increasing switching cycles. This erosion will enlarge the diameter of the openings by about 50% over 10.sup.7 cycles. This reduces the dielectric strength and increases the feed-through into the cathode rear space, which can result in increasing disturbances of the triggering system through overvoltages.
The present invention is directed to the problem of simplifying and improving the known design of a hollow-electrode switch. In particular, the invention seeks to simplify the trigger device for the hollow-electrode switch, to substantially increase the product p times d, and to reduce the voltage dependence of delay and jitter.